With the increasing fineness seen in semiconductor processes, there has also been a rapid increase in the fineness of photolithography. The early achievement of the schedule for increased fineness in the SIA roadmap has advanced year by year; this trend is accelerating, with an achievement one year ahead of schedule from the edition at the end of 1997 to the 1999 edition, and an achievement two years ahead of schedule in Scenario 2 of the 2000 edition. Accompanying this trend, there has been a yearly advance in the decrease of wavelengths of lithographic light sources. Specifically, beside the g line and i line of mercury lamps, exposure apparatuses that combine KrF excimer laser devices (λ=248 nm) or ArF excimer laser devices (λ=193 nm) with a refractive optical system (dioptric system) were used.
There is currently active research and development involving F2 laser exposure apparatuses that combine an F2 laser device (λ=157 nm) with even shorter-wavelength and a reflective-refractive optical system (catadioptric system) for use in the next generation of fine working at 100 to 70 nm node. Furthermore, an EUV exposure apparatus 10 that combines an extreme ultraviolet (EUV) light source with a wavelength of 13 nm and a reducing projection reflective optical system (cataoptric system) is expected for use in fine working at 50 nm node or below.
EUV exposure apparatuses will be described below. EUV exposure is a type of photolithography.
FIG. 5 shows one example of an EUV exposure apparatus 10 of the prior art. As is shown in FIG. 5, extreme ultraviolet light 13 with a wavelength of approximately 13 nm that is emitted from an extreme ultraviolet light source 11 inside a vacuum chamber (not shown in the figures) passes through a debris shield 12 and enters an illumination optical system 14. Furthermore, the term “debris” refers to debris that is generated by the extreme ultraviolet light source 11; the debris shield 12 prevents such debris from adhering to the optical elements.
The extreme ultraviolet light 13 that is shaped by a light-gathering mirror 15 is reflected by reflective mirrors 16, 16, and enters a reflective type mask (not shown in the figures) that mounted on the undersurface (in FIG. 5) of a reticle stage 17. A semiconductor circuit pattern is drawn on the reflective type mask, and the extreme ultraviolet light 13 enters a reducing reflective optical system 19 as an image of the semiconductor circuit pattern. As a result of being repeatedly reflected inside the reducing reflective optical system 19, the image of the semiconductor circuit pattern is reduced, and is focused as an image on the surface of a resist (not shown in the figures) that is coated onto a silicon wafer 20 carried on a wafer stage. As a result, an ultra-LSI circuit is formed.
Since the extreme ultraviolet light 13 has an extremely strong correlation with the substance used, a special structure is required in the reflective film of the reducing reflective optical system 19. Currently, in the case of an Mo/Si multi-layer film, a reflectivity of approximately 70% is obtained in a multi-layer film at the wavelength of 13 to 14 nm. Furthermore, in the case of a Be/Si multi-layer film, a reflectivity of close to 70% is obtained at the wavelength of 10 to 11 nm. However, since Be is very toxic, the realization of an extreme ultraviolet light source with a high brightness in the vicinity of 13 to 14 nm where a high reflectivity is obtained in an Mo/Si multi-layer film is desired.
Assuming that the throughput of an EUV exposure apparatus 10 is 80 frames/hour, and that the resist sensitivity is 5 mJ/cm2, a light source of 50 to 150 W is considered necessary for optical systems that are currently under consideration.
Furthermore, since the extreme ultraviolet light source 11 is a point light source or a collection of point light sources, the extreme ultraviolet light 13 must be in a range that allows the light to be gathered by the light-gathering mirror 15 of the illumination optical system and utilized. Specifically, in the propagation of the light beam of such point light source light, on the principle that the etendue is always constant, the etendue of the illuminated region (product of the area of the illuminated region and the steric angle of the illuminating light) must be smaller than the etendue on the side of the Light source (product of the area of the light source and the steric angle of diffusion).
If the etendue on the side of the light source is large, the light beam that cannot be taken into the illumination system increases. Accordingly, it is necessary to keep the etendue on the side of the light source at a small value; in order to accomplish this, however, the size of the light source must be sufficiently reduced. For example, in order to gather light from the light source at a steric angle of π, the diameter of the light source must be approximately 0.5 mm or less.
Furthermore, in order to ensure a uniform line width in the exposure pattern, it is desirable to use numerous pulses of illumination, and to control the amount of exposure by adding such pulses. A high repetition frequency is required for this purpose. Furthermore, in order to achieve accurate control of the amount of exposure, it is also necessary to suppress fluctuations in the individual pulses to a sufficiently small value.
An LPP (laser produced plasma) light source among various types of extreme ultraviolet light sources 11 will be described with reference to FIG. 6. This is a light source in which a plasma is produced by gathering light and illuminating a target 22 with a short-pulse laser, and the extreme ultraviolet light that is generated in this case is used as a light source. This LPP light source is an influential candidate for EUV exposure light sources in which a power of several tens of watts or greater is required.
In FIG. 6, short-pulse driving laser light generated by a driving laser device 25 is gathered and directed onto a target 22 that is caused to jet from a nozzle 21 inside a vacuum chamber (not shown in the figures). As a result, the target 22 is converted into a plasma, and extreme ultraviolet light 13 with a wavelength of ten odd nanometers is generated as a result of this conversion into plasma. Extreme ultraviolet light 13 with a relatively high output can be obtained by gathering this light with a concave mirror 34 or the like.
An LPP light source has the excellent features described in 1.1 through 1.5 below. Specifically:
1.1 Since the plasma density can be set at a high value, an extremely high brightness that is close to black body radiation is obtained.
1.2 The emission of light having substantially only the required wavelength band can be accomplished by selection of the target 22.
1.3 The light source is a point light source that has a substantially isotropic angular distribution, and there are no structures such as electrodes or the like around the light source.
1.4 The generation of impurities can be kept to a minimum.
1.5 An extremely large capture steric angle of 2π sterad can be ensured.
Currently, TRW Co. in the USA is developing an LPP light source in which an LD-excited YAG laser device (wavelength 1 μm) of the 1.5 kW class is caused to irradiate the target 22. When the target 22 is a solid target, a relatively high efficiency of 1 to several percent can be obtained as the conversion efficiency from laser light to the extreme ultraviolet light 13.
However, when the target 22 is a solid, it is difficult to convert all of the target 22 into a plasma. The target 22 that is not converted into a plasma is melted by the plasma at a temperature of several ten thousand degrees, and is released in large quantities as particle masses (debris) with a diameter of several μm or greater. This debris adheres to the surface of the concave mirror 34 that gathers the extreme ultraviolet light 13, and causes damage to the multi-layer film or the like, so that the practical utility of the LPP is conspicuously reduced.
On the other hand, when an Xe gas jet which is considered to show less debris than a solid is used as the target 22, the conversion efficiency from laser power to extreme ultraviolet light 13 is reportedly about 0.5%. Assuming that half of the extreme ultraviolet light 13 that is generated can be captured, then a laser device that has an extremely high output of 20 kW is required in order to obtain 50 W of extreme ultraviolet light 13 in the case of a gas target 22.
The selection of the target 22 and the method of supplying this target at a high density to the plasma generating position inside the vacuum chamber are the keys to achieve an increase in the output of the extreme ultraviolet light source 11. In concrete terms, the conditions described in 2.1 through 2.6 below are necessary.
2.1 The light emission efficiency in the vicinity of the desired wavelength (13 to 14 nm) must be high.
2.2 It must be possible to handle laser irradiation with a high repetition frequency.
2.3 Long-term continuous laser irradiation must be possible.
2.4 The plasma generating position and amount of plasma generated in each laser irradiation must all be maintained within a required precision.
2.5 The structure used must be able to efficiently capture the generated extreme ultraviolet light 13.
2.6 There must be little generation of debris.
In the past, tin (solid), xenon (gas), lithium (solid) and the like have been tried as materials of the target 22 that are suitable for light emission in the vicinity of 13 to 14 nm where a high reflectivity is efficiently obtained in an Mo/Si multi-layer film.
In particular, Xe is an inert gas that is chemically stable, and is at the same time a gas at ordinary temperatures; accordingly, since this material shows little adhesion to the mirror and few chemical reactions, as well as little generation of debris, Xe has attracted attention as an effective target 22, and has been studied.
In the past, systems such as those described in 3.1 through 3.7 below have been proposed and tried as supply methods when Xe is used as the target.
3.1 A gas jet system in which a high pressure is applied to Xe gas, and the gas is caused to jet into a vacuum from a nozzle 21.
3.2 A cluster jet system in which very small solid particles are created by the cooling effect of adiabatic expansion.
3.3 A spray system in which a liquid is sprayed from a nozzle 21.
3.4 An Xe pellet system in which solid Xe ice is dropped.
3.5 An Xe droplet system in which liquid Xe is dropped.
3.6 A system in which a laser is caused to strike a solid Xe ice mass.
3.7 A liquid filament system in which liquid Xe is caused to fly in a straight jet from a fine tube, and this jet is irradiated with pulsed laser light.
In particular, the liquid filament system of 3.7 appears to be the most advantageous system among systems for generating extreme ultraviolet light with a wavelength of 13 nm by means of Xe that have been reported so far.
Specifically, the density of the target 22 is increased by cooling xenon to a temperature below the boiling point (approximately minus 160° C.) so that this xenon is liquefied. Furthermore, the target 22 is supplied at a high density to a considerably distance (up to about 50 mm) from the nozzle 21 by relaxing the diffusion from the nozzle 21, and a plasma is generated by irradiating the target with laser light in this position.
Here, the distance between the nozzle 21 and the plasma generation position (laser light irradiation position) is defined as the working distance.
This liquid filament system has the technical advantages described in 4.1 through 4.6 below.
4.1 Compared to a gas-form target, the density can be increased to a high density that is close to the density of solid; accordingly, a high conversion efficiency is ensured.
4.2 The plasma generation point can be set at a long distance of 10 mm or greater from the nozzle 21, so that damage to the nozzle 21 caused by the heat of the plasma, and the resulting generation of debris, can be reduced.
4.3 The plasma generation point can be set in the center of the vacuum chamber, so that a high EUV light gathering efficiency is obtained.
4.4 The size of the plasma is small, so that the required etendue can easily be obtained.
4.5 The operation is continuous, and emission is accomplished by applying pressure to the liquid Xe; accordingly, there is no need for a driving mechanism.
4.6 The jet that is not converted into a plasma is in solid form, and is cooled and solidified by adiabatic expansion as the target 22 advances, so that recovery is easy.
However, the problems described in 5.1 through 5.6 below are encountered in abovementioned liquid filament system.
5.1 Hydrodynamic instability such as hose instability and the like resulting from the continuous jetting of a liquid from a fine tube tends to occur, so that the position of the target 22 oscillates spatially, thus making laser irradiation difficult.
5.2 It is difficult to supply the target 22 stably to a position that is removed from the nozzle. Accordingly, it is difficult to supply a target 22 with a large diameter, so that at present, only supply as a fine jet with a diameter of approximately 20 μm can be realized.
5.3 The light emission efficiency of the extreme ultraviolet light 13 relative to the power of the driving laser device 25 is relatively high. However, in order to increase the output of the extreme ultraviolet light source 11 to a high output, it is necessary to increase the diameter of the jet flow while increasing the power of the driving laser device 25 and maintaining the liquid jet characteristics “as is”. This is difficult as described in 5.2 above.
5.4 As was indicated in 5.2 above, the diameter of the jet is approximately 20 μm, and it is difficult to gather laser light with a high output in a stable manner in such a narrow region. For example, if the diameter can be increased to approximately 100 μm, the gathering of laser light is facilitated, so that the burden on the laser is reduced.
5.5 As a result of abovementioned 5.1, a mechanism for spatial stabilization of the jet is required.
5.6 As a result of the above conditions, a value of approximately 25 kHz is currently the limit for achieving an increase in the output by increasing the repetition frequency while maintaining stable conditions.
Furthermore, the stable conditions in abovementioned 5.6 are conditions which are such that the stability of the spatial position of the jet is maintained at approximately 1 μm, and the fluctuation of the density is kept to 1% or less. These conditions are necessary in order to achieve the value required as the stability of the EUV output.
Thus, in order to achieve an output that is sufficient for utilization in EUV exposure, it is desirable to use a system in which Xe in a liquid droplet state is produced, and this Xe is irradiated with laser light, as in the Xe droplet system in which liquid Xe is dropped in the form of droplets as indicated in abovementioned 3.5. If such a system is used, large liquid droplets of Xe are formed; accordingly, the output of the extreme ultraviolet light 13 that is generated can be increased.
However, in the case of the Xe droplet system, since the velocity of the Xe is slow, the distance between the plasma generation position and the nozzle 21 (abovementioned working distance) decreases as the repetition frequency is increased. As a result, the nozzle 21 may be damaged by the heat of the plasma.
In order to solve this problem, it is necessary to increase the working distance so that the plasma generation position is separated from the nozzle 21.
However, in the case of abovementioned droplet system, since the Xe does not have a sufficient velocity, the liquid droplets diffuse so that the density drops as the Xe becomes farther removed from the nozzle 21. As a result, the following problem arises: namely, extreme ultraviolet light 13 with a sufficient output cannot be obtained.